Semiconductor photodiodes are widely used for the detection of light, both visible and infrared. They exploit the internal photoelectric effect, where electron-hole pairs are generated in the semiconductor by photon absorption and contribute to electrical conduction inside the device, leading to a corresponding current at the contacts of the detector. Such detectors are fabricated singly, or in linear arrays for spectroscopy, or in two-dimensional (2-D) arrays for imaging.
To create highly sensitive detectors, low noise is desired. Low noise requires that all sources of leakage current in the photodiode should be suppressed to the greatest degree possible. Leakage currents in a semiconductor photodiode arise by a variety of mechanisms, including leakage at surface traps, leakage through bulk traps or defects, quantum-mechanical tunneling between the valence and conduction bands in the semiconductor, spontaneous electron-hole generation through thermal energy, impact ionization, and junction diffusion current.
Tunneling leakage can be reduced by employing moderate doping levels and low voltages. Bulk leakage can be reduced by using high-purity materials and by using growth techniques which avoid the formation of crystal defects such as stacking faults, twins, and dislocations. Spontaneous electron-hole generation and impact ionization are negligible in detectors made of an indirect bandgap material such as silicon or germanium. When all these leakage mechanisms have been reduced, surface leakage and diffusion current remain as the dominant leakage mechanisms.
Surface leakage is caused by traps at the interface between the semiconductor and any dielectric surfaces which contact it. The traps typically originate due to dangling bonds which result when the semiconductor lattice is terminated. Two types of surface leakage can be distinguished: leakage arising where a depletion region intersects a surface, and leakage where the semiconductor adjoining the interface is doped and charge-neutral. In both cases, leakage will arise whenever an electron-hole pair is generated at a trap on the surface, and the electron and hole make their way to different junctions, causing current to flow in an external circuit. Leakage at a depleted surface is proportional to the intrinsic carrier concentration and therefore depends on temperature as exp (−Eg/2kT) where Eg is the semiconductor bandgap. Leakage at a doped interface varies as exp (−Eg/kT) and is typically much lower. A semiconductor photodetector using the photoelectric effect, such as a P-N photodiode, cannot avoid having a depletion layer intersecting the semiconductor surface. The larger the depletion layer, the more surface leakage. The un-depleted surfaces will also give rise to leakage current even if some means is found to suppress leakage at the depleted surface.
Diffusion current is an intrinsic aspect of a diode and cannot be eliminated, though it can be reduced. It arises whenever voltage bias is applied to the diode. The applied voltage disturbs the minority carrier concentrations at the edge of the diode junction from their equilibrium values. The minority carrier concentrations at the contacts are always equal to their equilibrium values. Consequently there is a gradient of minority carriers between the junction and the contacts, giving rise to a steady diffusion current of minority carriers. Under reverse bias, the condition where a photodiode is normally operated, minority carriers flow from the contacts to the junction, where they are continuously swept away by the field to become majority carriers on the other side of the junction.
All these sources of leakage current compete with the photocurrent generated by incoming light, and therefore compete with the signal and reduce the signal-to-noise ratio.
Photodiodes formed in silicon exploit the highly optimized silicon/silicon dioxide surface. These surfaces, which have extremely low surface recombination velocities, are referred to as passivated surfaces. Such photodiodes are widely used in CCDs and CMOS imagers. However it is desirable to form photodetectors in other materials besides silicon, in order to form images using light of wavelengths to which silicon is not sensitive, e.g., infrared light.
Germanium is one material which can be used to form infrared-sensitive photodiodes. Germanium photodiodes have been reported to have undesirably high dark current for many applications. Reported leakage current densities for germanium diodes grown on silicon are of order 1 mA/cm2. See references designated [1][2] in the attached Appendix. This is approximately equal to the photocurrent that would be generated by bright sunlight, and represents a high level of leakage. Germanium photodiodes formed in bulk germanium have reported leakage 10-100 times lower [3][4], but this is still not sufficient for imaging indoors or in twilight conditions. To form low leakage detectors, improved devices and processes are needed.